1. Field of the Invention
The invention relates to curable coating compositions for substrates such as wood which are chemically bonded to the substrate.
2. Description of Related Art
It is generally known in the art that the surface properties of shaped articles such as polymeric films and fibers can be modified by graft polymerization processes wherein the article is treated with a free radical generating agent such as an organic peroxide or high energy radiation and then contacted with an ethylenically unsaturated monomeric material under conditions wherein the monomer or graft polymer chain is caused to be covalently bonded to the substrate.
One particular process involves the treatment of natural or synthetic polymer substrates containing active hydrogen with an aqueous solution containing a silver salt (silver nitrate), a free radical polymerization catalyst and a free radically polymerizable monomer. The silver salt acts upon the substrate to remove active hydrogen thereby creating active sites or free radicals along the molecular chain and initiating and propagating polymerization of the monomer in conjunction with the free radical polymerization catalyst. The resulting product is a graft copolymer comprising a substrate having a plurality of polymeric side chains covalently bonded thereto. Examples of such processes are disclosed in U.S. Pat. Nos. 3,401,049 and 3,698,931.
Although cellulose or wood is a substrate whose surface properties may be modified in accordance with the disclosure of the above referenced patents, this technology has not heretofore found application as means of providing a protective coating for wood products. The durability and weatherability of wood is limited because it is highly hydrophilic and is subject to attack by water, fungi and bacteria, particularly in outdoor applications. It is thus necessary to "seal" the wood by the application of protective coatings which form a barrier between the wood and the elements. The mere formation of grafted side chains on wood surfaces does not provide the degree of barrier protection needed to significantly prolong the life of the wood.
Typical protective coatings for wood include linseed oil, acrylic or polyurethane based paints and lacquers. However, these coatings are susceptible to abrasion, UV deterioration oxidation and permeation by moisture which limits their useful lifetime, particularly in outdoor applications. Also, adhesion to the wooden substrate may quickly deteriorate as a result of permeation by water, chemical attack, mildew, fungi and other causes.
Organic solvent-based coating compositions are known in the art based on compositions curable by condensation reactions and containing polymeric material such as hydroxy terminated polyesters, diesters, acrylics and alkyds and an amino or polyisocyanate crosslinking agent. These formulations are adapted to be applied to a substrate such as metal, heated to drive off the solvent and further heated at temperatures above 80.degree. C. to activate the crosslinking mechanism. As such, these protective coatings find few practical applications on wood substrates because of the necessity to employ heat to activate the crosslinking mechanism. An example of a coating which is curable by condensation and free radical reactions is found in U.S. Pat. No. 5,087,690.
Wood coating formulations which can be cured without heat are also known in the art. For example, U.S. Pat. No. 4,171,387 discloses protective and decorative coatings for cellulose (wood) substrates based on a polyurethane base coat applied as a solution in organic solvent, which is further overcoated with an ultraviolet curable polyacrylic resin containing a photo-initiator. However, such coatings are also of limited interest due to the need to expose the coated surface to ultraviolet radiation to cure the composition.
Other coating formulations which may be cured at ambient or elevated temperatures are disclosed in U.S. Pat. No. 4,303,563. These are organic solvent based formulations containing an addition polymer which includes pendant primary or secondary amine groups and, as a crosslinking component, one or a mixture of monomers or prepolymers containing at least two unsaturated acryloxy groups.
Despite these developments, there exists a need to develop coatings, particularly coatings for wood, which can be cured at ambient temperatures, can be formulated without added organic solvents and which exhibit good initial and prolonged adhesion to the substrate and seal the substrate.